The purpose of variable direction of view endoscopes is to provide greater viewing mobility. Variable direction of view endoscopes come in two general classes: rigid and flexible. Rigid variable direction of view scopes either change their line of sight mechanically, as disclosed in U.S. Pat. No. 3,856,000 to Chikama, U.S. Pat. No. 6,371,909 to Hoeg, U.S. Pat. No. 6,560,013 to Ramsbottom, U.S. Pat. No. 4,697,577 to Forkner, U.S. Pat. No. 6,500,115 to Krattiger et al., and U.S. Pat. No. 5,762,603 to Thompson, or electronically, as disclosed in U.S. Pat. No. 5,313,306 to Kuban, and U.S. Pat. No. 5,800,341 to McKenna et al. The first advantage with these designs is that they allow wide range scanning without having to move the endoscope, making them ideal for inspecting small cavities. The view vector can be rotated about the longitudinal axis of the scope (pan), and about an axis normal to the endoscope shaft (tilt) to cover a large solid angle. Typically, these two degrees of freedom are rigidly actuated from the proximal end of the endoscope. Because these scopes rigidly transmit motion from the operator to the view vector, the view is relatively easy to control, a key advantage.
These rigid mechanisms also have the advantage of being kinematically determinate, which means that there is a known mathematical relationship between the distal and proximal ends. That is, based on the structure of the endoscope there is an available linear transformation (in robotics called the Forward Kinematic Transformation) which can be used to compute the endoscopic viewing configuration based on input from proximal accelerometers and encoders. This transformation makes it possible to implement electronic control, as disclosed in U.S. Pat. No. 6,663,559 to Hale et al. who teach the use of computer-controlled actuators and sensors to position the view vector and thus provide the endoscope with advanced navigation capabilities. Along related lines U.S. Pat. No. 5,524,180 to Wang et al. teaches a robotic positioning system for endoscopes, and U.S. Pat. No. 6,097,423 to Chatenever et al proposes the use of sensors and actuators to control the endoscopic image orientation. The successful operation of all these systems relies on having a known mathematical relationship between the distal and proximal ends of the instrument.
Unlike rigid scopes, flexible variable direction of view endoscopes change their viewing direction by deflecting the entire distal section of the instrument. Their main advantage over rigid scopes is that they can reach certain areas not accessible by rigid instruments. Otherwise they are inferior in the following ways:                Restricted mobility. Because they must flex in order to change their line of sight, mobility is restricted in small cavities. Specifically, the tip can not actuate properly if it is too close to a wall.        Unknown viewpoint. It is often difficult for the user to stay oriented when using flexible endoscopes because the view point displaces itself in a confusing way as the tip is flexed. Kanehira (U.S. Pat. No. 3,880,148) tried to solve these problems by incorporating a pivotable distal reflector as seen in rigid variable direction of view scopes, but the design was too complex, and it suffered from another fundamental shortcoming of flexible endoscopes: shaft wind-up.        Shaft wind-up. A flexible shaft cannot predictably transmit twisting motions. Unlike rigid variable direction of view endoscopes which can rely on rigid transmission of a torsional moment through their shaft to obtain the necessary second degree of view vector freedom at the tip, flexible endoscopes tend to wind up and do not reliably transmit twisting motions from the proximal to the distal end. This is because the surrounding tissue exerts sufficient frictional force to counteract twist force from the operator. This problem is so severe that makers of flexible endoscopes eventually designed four-way tip deflection mechanisms in an attempt to regain some of the mobility lost through shaft wind-up. Stored torsional energy can also cause a wound scope to release unexpectedly, possibly injuring tissue.        Indeterminate kinematics. Besides reduced controllability from twisting and bending, the flexible nature of the endoscope shaft introduces a further complication: the kinematic relationship between the distal and proximal ends is unknown. Because a flexible shaft accommodates the geometry/topography of its surroundings, the configuration of the endoscope tip and its view vector can not be correctly computed from sensor inputs at the proximal end. This means that unless the tip or shaft is instrumented with sensors, it is impossible to provide advanced navigation features (such as those described by Hale et al.).        Decoupled viewing process. Another drawback resulting from the mathematical decoupling between the distal and proximal ends is that the scope's viewing direction can not be mentally visualized by the operator during use. Getting an external estimate of where an endoscope is “looking” during a procedure is important as the clinician tries to integrate preexisting knowledge of the anatomy with the viewing process. With rigid fixed-viewing endoscopes the operator can extrapolate the viewing direction as an extension of the longitudinal axis of the endoscope even though the tip is concealed by surrounding tissue. With rigid variable viewing direction instruments the configuration of the view vector is kinematically determinate and so can be presented graphically to the user (Hale et al). Neither of these advantages is possible with current flexible endoscopes.        
The results of these shortcomings are poor controllability, operator disorientation, inconsistent diagnoses from one operator to the next, disjunct viewing coverage, reduced ability to produce diagnostic maps, and difficulty returning to a previous view for biopsy.
Accordingly, the primary object of the present invention is to provide a flexible endoscope with the capabilities and advantages of a rigid variable direction of view endoscope. Another object of the present invention is to make flexible endoscopes more compatible with computer-aided navigation systems. Still further objects and advantages will become apparent from the ensuing description and drawings.